British prime minister David Cameron says troops could start withdrawing from Afghanistan as early as next year.

He made the comments during an unannounced trip to the war-torn country.

Mr Cameron, who arrived on Monday, shrugged off US and Afghan criticism of the British troops' performance, saying it no longer held true.

He was speaking after a US diplomatic cable released by WikiLeaks said British troops deployed in the Taliban stronghold of Helmand were "not up to the task" of securing the province.

At a conference in Lisbon last month, NATO leaders agreed to meet Afghan president Hamid Karzai's timeline for foreign troops to end combat operations in Afghanistan by the end of 2014.

Some US and NATO leaders have warned that may spill into 2015, throwing the spotlight on the readiness of Afghanistan's 260,000 police and soldiers to take over from foreign forces.

But Mr Cameron and the head of Britain's armed forces, General David Richards, say improving conditions could allow British troops to start withdrawing next year.

US and NATO commanders have been talking up improvements in Afghan forces, although others acknowledge there are problems with the training, equipment and retention rates and that a target of 306,000 Afghan forces by October 2011 might be hard to meet.

Speaking at a joint press conference with Mr Karzai in Kabul, Mr Cameron said "real progress" made this year must be made "irreversible" in 2011.

"There's no scope for complacency... but I'm cautiously optimistic we have the right strategy," he said.

"We are now a year into that strategy. We have put in the right resources to back it up.

Britain has about 9,500 troops in Afghanistan, the bulk of which are in Helmand, where they were spread thinly until US president Barack Obama last December ordered an extra 30,000 troops to Afghanistan, the last of which arrived in August.

British troops have been able to concentrate on smaller, strategic areas of Helmand since the extra US troops arrived, officials said.

Mr Obama is due to review his Afghanistan war strategy this month, with civilian and military casualties at their highest level since the Taliban was ousted in 2001 despite the presence of about 150,000 foreign troops.

At least 346 British troops have died in Afghanistan since 2001, almost a third of them this year.